ssh: return ServerAuthError after too many auth failures

if a client is disconnected due to too many authentication attempts
we should return a ServerAuthError instead of a generic error.

Some users check the error returned by NewServerConn to determine
whether or not a client attempted to authenticate.

Fixes golang/go#69191

Change-Id: If68fcecdefd6c810fe9df8256b1216e320d8a916
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/566398
Reviewed-by: Filippo Valsorda <[email protected]>
Reviewed-by: Tim King <[email protected]>
Auto-Submit: Nicola Murino <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

ssh/client_auth_test.go

@@ -641,17 +641,28 @@ func TestClientAuthMaxAuthTries(t *testing.T) {
 		defer c1.Close()
 		defer c2.Close()
 
-		go newServer(c1, serverConfig)
-		_, _, _, err = NewClientConn(c2, "", clientConfig)
-		if tries > 2 {
-			if err == nil {
+		errCh := make(chan error, 1)
+
+		go func() {
+			_, err := newServer(c1, serverConfig)
+			errCh <- err
+		}()
+		_, _, _, cliErr := NewClientConn(c2, "", clientConfig)
+		srvErr := <-errCh
+
+		if tries > serverConfig.MaxAuthTries {
+			if cliErr == nil {
 				t.Fatalf("client: got no error, want %s", expectedErr)
-			} else if err.Error() != expectedErr.Error() {
+			} else if cliErr.Error() != expectedErr.Error() {
 				t.Fatalf("client: got %s, want %s", err, expectedErr)
 			}
+			var authErr *ServerAuthError
+			if !errors.As(srvErr, &authErr) {
+				t.Errorf("expected ServerAuthError, got: %v", srvErr)
+			}
 		} else {
-			if err != nil {
-				t.Fatalf("client: got %s, want no error", err)
+			if cliErr != nil {
+				t.Fatalf("client: got %s, want no error", cliErr)
 			}
 		}
 	}

ssh/server.go

@@ -510,8 +510,8 @@ userAuthLoop:
 			if err := s.transport.writePacket(Marshal(discMsg)); err != nil {
 				return nil, err
 			}
-
-			return nil, discMsg
+			authErrs = append(authErrs, discMsg)
+			return nil, &ServerAuthError{Errors: authErrs}
 		}
 
 		var userAuthReq userAuthRequestMsg